Wireless telecommunications systems often require high power electronics enclosed in a weather resistant electronic unit which may be deployed remotely, for example, at the top of a mast or tower near an antenna. Such units may be base transceiver stations for wireless communications, for example GSM band wireless communications used in North America and Europe. Mounting of one or more power amplifier units of high power transceiver stations on the mast head close to the antenna is desirable to minimize the losses in the cable from the amplifier to the antenna. Power amplifiers for such applications have high distributed power, typically .about.200 W or more. Several high thermal flux components, 10 W to 50 W, may be mounted on individual circuit boards within the unit. Consequently forced air cooling is typically required for warm weather operation to maintain components at a typical maximum transistor flange temperature of 85.degree. C. for a 50.degree. C. ambient atmospheric temperature.
Units for outside plant telecommunications equipment are required to operate over a wide range of weather conditions and temperatures in uncontrolled environments. Depending on climate, extreme ambient temperatures may be the range from -40.degree. C. to +50.degree. C. The enclosure is required to provide protection from the elements. It may include thermal insulation, sealing or other means to control moisture and condensation.
Conventional high power units require cooling in warm weather conditions and heating during cold weather operation to ensure that the operating temperature is maintained in an useful range.
For example, during warm weather, cooling using forced convection may rely on the use of fans or other air movers. Some outside plant, such as CATV cabinets use air conditioners. Air conditioners, fans or other active electronics often result in problems with reliability and servicing requirements for remote or antenna mounted units. Both cooling units and heaters add to the power dissipation, size and weight of the unit.
Conventional heat sink technology, in which a relatively massive heatsink forms a base plate of the enclosure for distribution of heat, can provide high thermal dissipation. For example, a power amplifier assembly capable of dissipating 1100 W in a volume of 0.02 cubic metres is described in U.S. Pat. No. 5,352,991 issued October 1994 using conventional heatsink technology and forced air convection. However a conventional heatsink assembly adds significantly to the size and weight of the enclosure. The relatively large size of the extrusion or casting required for an integral heat sink for power dissipation at this level also adds to manufacturing and installation challenges. Known bonded fin heatsinks overcomes some practical manufacturing problems, but still require a structure which is of relatively large and heavy construction. These large, heavy units are particularly inconvenient for installation and servicing in remote locations, or relatively inaccessible locations, such as antenna masts.
Other known means of heat sinking include the use of heatpipe assemblies. For example, thermal management of power amplifier devices mounted on a metal chassis providing an RF enclosure has been described in U.S. Pat. No. 4,963,833 issued Oct. 16, 1990 entitled `High powered amplifier and heat sinking apparatus` using a heat sinking device including a heatpipe associated with each power device, which relies direct contact of the heat sinking device with an exposed area of the power amplifier.
U.S. Pat. No. 4,673,030 issued Jun. 16, 1987 entitled `Rechargeable thermal control system` is an earlier example describing the use of a unidirectional heatpipe as a thermal diode or heat switch in a thermal control system.
A related copending U.S. patent application Ser. No. 08/812 831 filed Mar. 6, 1997, with a common co-inventor with the present application, and to the same assignee, is entitled "Electronic Unit". This application discloses the use of a heatpipe and cooling fin assembly for passive thermal control of an electronic unit, e.g. for telecommunications outside plant housing one or more circuit boards. This structure relies on heat conduction through ends of thermally conductive circuit boards to the heatspreader and heatpipe assembly. Consequently, power dissipation is limited by thermal conduction through ends of the circuit boards. This configuration is not readily adaptable for existing high power RF amplifiers circuit assemblies for high power wireless base-stations mentioned above.